1. Field of the Invention
The present invention relates to a mobile communication system and a radio resources assigning method for the system, and a base station, and more specifically to a mobile communication system and a radio resources assigning method for the system, and a base station that divide a transmission frequency band into a plurality of frequency blocks (RBs: frequency blocks), and assigns (schedules) radio resources to each mobile station for each RB.
2. Description of the Related Art
If the frequency band used in a radio transmission is relatively narrowband, it is assumed that the reception level in the band is constant. However, if it is broadband, the frequency dependency at a reception level is outstanding, and it is not appropriate to approximate it by a constant value.
Therefore, this type of mobile communication system uses a radio resources assigning method in which the transmission frequency band is divided into a plurality of frequency blocks (RBs), and the radio resources are assigned to each mobile station for each RB.
FIG. 6 shows the configuration of an example of the related mobile communication system. In FIG. 6, an example of the related mobile communication system includes an upper layer 101, a base station 102, one or more mobile stations 103-1 to 103-n (n is a positive integer).
The base station 102 includes a scheduler 111, a data channel decoding portion 112, a control channel decoding portion 113, a control channel generation portion 114, a data channel generation portion 115, a pilot channel generation portion 116, a demodulation portion 117, a channel estimation portion 118, a modulation portion 119, a radio portion 120, and an antenna 121.
The mobile stations 103-1 to 103-n respectively include antennas 131-1 to 131-n.
The data channel generation portion 115 of the base station 102 converts data 201 transmitted from the upper layer 101 into a data format matching the radio channel, error-correction-codes the data, and transmits it to the modulation portion 119. The conversion includes the process of converting data into a radio frame size or a multiple of the size.
The control channel generation portion 114 receives control channel information (modulation system of a data channel, parameter relating to a coding rate, occupied RB number, etc.), error-correction-codes the data, converts the data into the data format of the control channel, and outputs the resultant data as data 203 to the modulation portion 119.
The pilot channel generation portion 116 generates a pilot pattern 204 and outputs it to the modulation portion 119.
The modulation portion 119 assigns the data 203 of the control channel, data channel transmission data 202, pilot channel transmission data 204 to each RB in the transmission signal based on the RB assignment information 205 from the scheduler 111, forms a transmission signal 206, and outputs it to the radio portion 120. The formation of the transmission signal 206 includes the process of modulating and spreading each data.
The radio portion 120 has the function of converting the transmission signal 206 into a signal of the RF band, and outputting the signal as a transmission signal 207 to the antenna 121, and the function of converting the received signal of the antenna 121 into a baseband signal 208 and outputting it to the demodulation portion 117. The signal 207 represents both a transmission signal and a received signal of the RF band.
The channel estimation portion 118 obtains a channel estimation value by the despreading process from the received signal 208, and outputs it as data 209 to the demodulation portion 117.
The demodulation portion 117 demodulates the data of the data channel from the received signal 208 based on the data 209 indicating the channel estimation value and data 210 indicating a control channel decoding result, and generates a soft determination value 211.
The demodulating function includes the despreading and the demodulation calculation on the received data. The demodulation portion 117 demodulates the control channel data from the received signal 208 based on the data 209 indicating the channel estimation value, and calculates a soft determination value 212.
The data channel decoding portion 112 has the function of error-correction-decoding the soft determination value 211 of the data channel. Decoded data 213 is output to the upper layer 101.
The control channel decoding portion 113 error-correction-decodes the soft determination value 212 of the control channel, and outputs the result as data 214 to the scheduler 111.
On the other hand, disclosed is a radio transmission system for measuring the reception quality (signal to interference ratio: SIR) for each radio frame, measuring the amount of the fluctuation of the reception quality from the measurement value, and selecting the modulation mode depending on the amount of the fluctuation of the reception quality (for example, refer to the patent document 1 (Japanese Patent Laid-Open No. 2005-136773 (paragraphs 0006, 0011, 0012, 0016, and FIG. 2)).
Furthermore disclosed is the invention relating to the frequency scheduling for dividing the entire frequency band assigned to the system into a plurality of frequency blocks and assigning radio resources for each divided frequency block (for example, refer to the patent documents 2 and 3 (Japanese Patent Laid-Open No. 2006-050545 (paragraphs 0004, 0015, 0069, and FIG. 2), and Japanese Patent Laid-Open No. 2006-237897 (paragraphs 0002 to 0004, 0014, 0114, and FIG. 3)).
Additionally disclosed is a radio communication device for dividing blocks depending on the size of the SIR and determining the appropriate modulation system depending on the SIR for each frequency block (for example, refer to the patent document 4 (Japanese Patent Laid-Open No. 2006-287758 (paragraphs 0003, 0022 to 0024, and FIG. 2)).
However, in this type of related mobile communication system, when a base station determines the assignment of resources to an RB, the fluctuation in the time direction of the SIR of the mobile station is not taken into account, but the measurement value or the average value of the SIR is considered. Therefore, in the propagation environment in which there arises a sharp drop in the SIR such as fading, the resources cannot necessarily be assigned to optimize the entire throughput.
That is, in this type of related mobile communication system, when it is determined (scheduled) to which RB the data of each mobile station is assigned, the determination is based on the downlink SIR that is received by each mobile station or the average value of it.
Each mobile station has the function of calculating the SIR (or a plurality of RBs are grouped, and one SIR is assigned to each group) for all RBs in the system available bands, and notifies the base station of the calculation result through the uplink control channel.
FIG. 7 is a graph showing an example of the relationship between the related frequency block (RB) and the SIR average value. FIG. 8 is a graph showing an example of the relationship between the time and the frequency of the related frequency block (RB).
For example, as shown in FIG. 7, when the SIR in the mobile station 1 is relatively high in the RB 0 and the RB 1, and the SIR in the mobile station 2 is relatively high in the RB 2 and the RB 3, the data addressed to the mobile stations 1 and 2 is respectively assigned to the RB 0 and 1, and the RB 2 and 3 so that the optimum throughput of the entire system can be obtained. As a result, as for the format of the radio frame, the data addressed to the mobile stations 1 and 2 assigned to the RB 0 to 3 as shown in FIG. 8 is simultaneously transmitted.
However, in the fading environment, the entire throughput cannot be necessarily optimum by assigning the RB only based on the average SIR. Because the SIR exhibits a sharp drop in the fading environment, there is such data that cannot be corrected by an error correction code, thereby reducing the throughput. But even in such a case, there can be low SIR degradation in average SIR value if a relatively long time period can be reserved with a preferable SIR.
That is, as disclosed by the related technology, if only an average SIR of each mobile station is counted, the occurrence of a decoding error in a mobile station by a sudden drop in the SIR caused by fading, that is, the reduction of throughput, cannot be easily reflected when an RB is assigned.
Any of the above-mentioned patent documents 1 to 4 does not disclose effective means to solve the problems above.